The EGR (Exhaust Gas Re-circulation) method is known as a technology that is used for reducing problematic NOx (nitrogen oxide) in exhaust gas emitted from a diesel engine. On the other hand, when the EGR method is applied, the amount of a fresh air (a fresh intake air flow rate) inhaled by the engine decreases relatively which is prone to cause an O2-deprived (oxygen-deprived) atmosphere in a combustion chamber of the engine when the engine is rapidly accelerated or a fuel admission opening of the engine is rapidly increased.
In order to overcome the above-mentioned difficulty, increasing the amount of the fresh intake air by constraining EGR gas flow rate seems to be a possible countermeasure. However, since control response speed as to an EGR system is slower, controlling the EGR gas flow rate is not enough, and thus, it is required to provide a countermeasure as to a fuel injection system control which has a faster control response speed.
However, it must be taken into consideration that the fuel admission (accelerator opening) control independent of engine load control or engine speed control may hinder the engine speed stability. In other words, ensuring compatibility between the exhaust gas performance and the engine speed response performance is a technical prerequisite.
For instance, a patent reference 1 (JP1999-36962) discloses a fuel admission control method by which the amount of exhaust gas smoke of a diesel engine produced when accelerated does not exceed a target density. In the method of the reference 1, a maximum fuel mass quantity in relation to the fresh intake air flow rate and the engine rotation speed, whereby the exhaust gas smoke is not produced, is set as a map in advance, and with the determined maximum fuel mass quantity based on the detected fresh intake air flow rate and the detected engine rotation speed, production of the exhaust gas smoke is controlled.
Further, a patent reference 2 (JP1997-151761) discloses an injection annealing control in which the fuel flow rate is gradually increased with a constant increment to a constant increment, while an EGR valve opening is controlled, so that the exhaust gas smoke due to the slow response of the EGR system is not produced during the engine acceleration, and the performance of the engine acceleration is kept satisfactory. However, in the technology of the patent reference 1, a lot of man-hours are required in creating the above-mentioned map that prescribes a maximum fuel mass quantity; on the other hand, in the control technology of the patent reference 2, there is no consideration as to residual O2 (oxygen) in the EGR gas that returns back into the combustion chamber (or the intake air system of the engine) from the engine exhaust system, although the residual O2 (or the residual air that is not used in burning fuel) that has not been consumed in the former engine combustion stroke has an essential effect on the smoke yielding in the next combustion stroke.